Recently, laser marking devices for projecting a straight line at a wall or the like with a laser beam have been used in a marking operation for drawing a reference line in construction sites. For example, Japanese unexamined Patent Application Publication No. 10-141957a discloses a laser marking device configured so that a lens tube member that mounts a laser optical system through a gyro mechanism at its base is supported in a suspended state and the laser optical system can emit a ceiling face projecting beam, a floor face projecting beam and a side wall projecting beam to a ceiling face, a floor face and a side wall, respectively, at the same time. The laser optical system includes a single laser light source, a polarizing means for polarizing a laser beam emitted from the laser light source into an upward vertical beam, a downward vertical beam and a horizontal beam and a vertical axis adjusting means for adjusting the ceiling face projecting beam and the floor face projecting beam on a single vertical axis. The lens tube member is horizontally suspended by the gyro mechanism.
Japanese unexamined Patent Application Publication No. 2003-329816 discloses a polarizing means obtained by bonding or sticking a plurality of members made of glass or plastic that passes light therethrough together so as to be shaped like a rectangular parallelepiped as a whole.
In the conventional laser marking devices, a laser light source that emits red laser light is generally used. However, when such light source is used in an outdoor or brightly lit construction site, a line beam is disadvantageously hard to be seen and thus, a laser marking device that uses green laser light having excellent visibility has been considered. At this time, since the light source using green laser light is large and expensive, as disclosed in Japanese unexamined Patent Application Publication No. 10-141957a, etc., it has been considered to divide a beam emitted from a single light source into a plurality of laser beams.
However, it is found that, when the green laser light source is used, the intensity of each laser beam that is reflected on or passes through the polarizing means (splitters) disclosed in Japanese unexamined Patent Application Publications No. 10-141957a and No. 2003-329816 varies depending on a change in the polarization characteristics of the laser beam according to temperatures, impairing the usability of the laser marking device. Further, the splitter disclosed in Japanese unexamined Patent Application Publication No. 2003-329816 can be broken, when the splitter is fixing at the lens tube member.